A combined structural and functional study of the canine renal lymphatic system is in progress. The morphological approach includes light and electron microscopic studies of the detailed intrarenal distribution and relationships of lymphatic capillaries. Work so far has been primarily concerned with the capsular system, intralobular cortical lymphatics and communications between the capsular and hilar systems. Intralobular lymphatics have been identified mainly in relationship with elements of the arterial and arteriolar systems, although some examples related only to tubular elements have been seen. Tributaries to the capsular system were found to be of two types - a communicating, which primarily connected the capsular and hilar systems and a perforating which drained a subcapsular plexus. Tracer studies using ferritin and horseradish peroxidase and employing instantaneous in vivo drip fixation have demonstrated the pathway taken by large molecules through the blood capillary endothelium into the interstitium. Further tracer studies are being used to delineate pathways through the interstitium to lymphatic vessels. Functional studies involve analysis of renal lymph, plasma and thoracic duct lymph under differing conditions of renal function. Under control conditions hilar lymph Na ion and Cl' concentrations exceed those in simultaneous plasma. These concentration differences were found to be relatively unaffected during isotonic, hypotonic and hypertonic volume expansion in spite of considerable alterations in plasma levels. The results were consistent with an outer medullary contribution to hilar lymph. The cellular content of peripheral renal lymph is also under study. BIBLIOGRAPHIC REFERENCES: Holmes, M.J., P.J. O'Morchoe and C.C.C. O'Morchoe. Structural and Functional Alterations in the Lymphatic Drainage of the Kidney During Ureteric Occlusion. Anat. Rec., 184, 430, 1976. C.C.C. O'Morchoe, P.J. O'Morchoe, M.J. Holmes and H. Jarosz. Relationship Between Na ion and Cl- Levels in Renal Plasma and Lymph. Proc. Am. Soc. Neph. 1976 Kidney Int. (In Press).